ASSESSMENT OF Fe
2O
3/SiO
2CATALYSTS FOR THE
CONTINUOUS TREATMENT OF PHENOL AQUEOUS
SOLUTIONS IN A FIXED BED REACTOR
J.A. Botas*, J.A. Melero, F. Martínez, M.I. Pariente
Department of Environmental and Chemical Technology, ESCET, Rey Juan Carlos University,
c/Tulipán s/n, 28933, Móstoles, Madrid, Spain
Published on:
Catalysis Today 149 (2010) 334–340
doi:10.1016/j.cattod.2009.06.014
Keywords: iron catalyst; mesostructured silica; phenol; fixed bed reactor; CWHPO
Abstract
Different iron-containing catalysts have been tested for the oxidation of phenol aqueous
solutions in a catalytic fixed bed reactor in presence of hydrogen peroxide. All the catalysts
consist of iron oxide, mainly crystalline hematite particles, over different silica supports
(mesostructured SBA-15 silica and non-ordered mesoporous silica). The immobilization of iron
species over different silica supports was addressed by direct incorporation of metal during the
synthesis or post-synthesis impregnation. The synthesis conditions were tuned up to yield
agglomerated catalysts with iron loadings between 10 and 15 wt. %. The influence of the
preparation method and the type of silica support was evaluated in a catalytic fixed bed reactor
for the continuous oxidation of phenol in terms of its activity (phenol and total organic carbon
degradation) as well as its stability (catalyst deactivation by iron leaching). Those catalyst
prepared by direct synthesis either in presence (Fe2O3/SBA-15(DS)) or absence
(Fe2O3/SiO2(DS)) of template molecules achieved high catalytic performances (TOC reduction
of 65 and 52 %, respectively) with remarkable low iron leaching in comparison with the
impregnated iron catalysts. Catalytic results demonstrated that the synthesis method plays a
crucial role in the dispersion and stability of active species and hence resulting in superior
1. Introduction
The degradation of aromatic pollutants in wastewater streams generated by industrial
processes has emerged as an important concern due to the restrictive environmental laws and
regulations. Phenol and phenol-like compounds show high toxicity for microorganisms, high
chemical oxygen demand and poor biodegradability. Advanced Oxidation Processes (AOPs) are
postulated as interesting alternatives for the destruction of organic pollutants in industrial
wastewaters. These processes involve the generation of non-selective and highly reactive
hydroxyl radical (●OH), which is one of the most powerful oxidation agents [1], higher than
others chemical oxidants commonly used in wastewater treatments.
The use of hydrogen peroxide as oxidant is an option commercially available for the
treatment of industrial wastewaters [2,3]. Nevertheless, the hydrogen peroxide by itself is not
effective at high concentrations of refractory compounds due to the reaction rate is very low,
even for high oxidant concentrations [4]. In order to promote the generation of hydroxyl radicals
and reduce secondary reactions, the use of catalytic processes, such as Fenton systems, could be
interesting alternatives. The reaction of hydrogen peroxide with ferrous salts (Fenton’s reagent)
or other low-valence transition metals (Fenton-like reactions) at room temperature are well
known sources of hydroxyl radicals [5]. In some cases, temperature has been increased up to
80-120 ºC leading to the so-called Catalytic Wet Hydrogen Peroxide Oxidation (CWHPO)
processes. The homogeneous oxidation of organic compounds by means of Fenton-like
reactions has been studied in numerous works during last years and it is commercially used to
treat industrial wastewaters [6]. However, these homogeneous systems present two main
drawbacks. Firstly, the hydroxyl radical production by Fenton-like reactions is strongly
dependent on limited pH range (2.5-3.5) and secondly, a final separation of soluble iron species
from the treated wastewater is needed.
These drawbacks have promoted the development of Fenton and CWHPO processes
based on heterogeneous catalytic systems. The catalyst design is usually a key factor in order to
increase its surface area, minimize the metal sintering, improve its chemical stability and govern
incorporation of active iron species over different supports, such as silica, zeolites [7-8],
hexagonal mesostructured materials (MCM-41, HMS 9 and SBA-15) [9-13], as well as pillared
clays [14] for its application in CWHPO. An interesting overview of solid catalysts is reported
by Perathoner and Centi [15], in which Al-Fe-clays are shown as one group of the most
promising catalysts. More recently, the same authors have stated that copper pillared clays are
highly effective for WHPO of wastewater streams from olive oil milling production with low
loss of copper by leaching [16]. Melero et al. [11] have also reported a remarkable catalytic
performance of a novel Fe2O3/SBA-15 composite catalyst as compared to homogeneous systems
as well as a low dependence on the pH for the treatment of phenolic aqueous solutions.
These heterogeneous processes have been carried out in batch operations and phenol is
frequently used as model pollutant. The use of solid catalysts for CWHPO indicates a good
perspective of this technology for its application in continuous processes, but it still requires
large engineering efforts in modelling and optimal designing of catalyst and industrial reactors.
Moreover, their application in continuous systems is normally questioned by the catalyst
fouling, in particular for the treatment of wastewater with a complex composition. On the other
hand, for its application in fixed bed reactors, catalysts must be shaped into bodies such as
granules, spheres and extrudates, in order to decrease the pressure drop and to achieve a
convenient mechanical strength. However, there is almost no information in the open literature
about its agglomeration process, although this is an essential step regarding its commercial
application. Therefore, few works are described in literature dealing with the treatment of
organic pollutants by means of a fixed bed reactor using hydrogen peroxide as oxidant. In this
sense, a major experience of continuous processes can be found in catalytic wet air oxidation. In
this context, the aim of this contribution has been the study of the influence of several supports
and iron incorporation methods for the degradation of phenol aqueous solutions using a
2. Experimental
2.1. Preparation of powder and extruded Fe/SiO2 materials
Several powder silica-supported iron materials were prepared following two different
methods: direct incorporation of the metal during the synthesis and post-synthesis incipient
wetness impregnation. In the first strategy, iron was incorporated by direct co-condensation of
the silica (TEOS) and iron (FeCl3·6H2O) sources during the synthesis of: i) mesostructured
SBA-15 materials with hexagonal mesoscopic order (Fe2O3/SBA-15(DS)) following the method
described elsewhere [17] and ii) non-ordered silica support synthesized by means of the sol-gel
method, based on acid-catalysed hydrolysis and basic condensation with ammonium hydroxide
(Fe2O3/SiO2(DS)). The second method based on the incipient wetness impregnation with
Fe(NO3)3·9H2O aqueous solution was carried out over: i) silica SBA-15 material (Fe2O3
/SBA-15(I)), SiO2 sol-gel (Fe2O3/SiO2(I)) and commercial silica from PQ Corporation (FeI/SiO2
-PQ(I)). With the purpose of comparison, a physical mixture of hematite iron oxide (Fe2O3;
Aldrich) with the mesostructured SBA-15 silica (Fe2O3+SBA-15(PM)) was also prepared. Prior
to the agglomeration of the powder catalysts, all the materials were calcined at 550 ºC for 5
hours.
Fe/SiO2 extrudates were prepared by blending of the powder catalyst with sodium
bentonite (75:25 wt %) and synthetic methylcellulose polymer (10 wt. % of the previous
mixture) which act as binders in the extrusion process [18]. All the components were kneaded
with deionised water under high shear conditions until obtaining a homogeneous paste, which
was conditioned in a damp atmosphere. Thereafter, the paste was pushed through a ram extruder
of 4.8 mm circular die and the resultant rod-shaped materials were dried for 3 days in a climatic
chamber under controlled temperature and relative humidity (RH) from 20 ºC and 70 % RH to
40 °C and 10 % RH. Finally, the extrudates were calcined in air until 650 °C (2 hours) using a
slow ramp of temperature, in order to achieve a gradual removal of water and the organic
content, strengthening the mechanical consistence of the material by sintering of the inorganic
binder. The final pellets for the catalytic reactions were crushed and sieved until particle sizes
2.2. Catalysts characterization
Physicochemical and textural properties as well as the presence of crystalline phases of
the powder materials and extruded catalysts were analyzed by means of different conventional
techniques. X-ray powder diffraction (XRD) data were acquired on a Philips X-Pert
diffractometer using Cu Kα radiation. The data were collected ranging 2θ from 0.5 to 50º with a
resolution of 0.02º. Nitrogen adsorption and desorption isotherms at 77 K were measured using
a Micromeritics Tristar 3000 System. Transmission electron microscopy (TEM) images were
taken on a Philips Tecnai-20 electron microscope operating at 200 kV. SEM images were
obtained in a XL30 ESEM FEI (PHILIPS) electron microscope. The iron content of the
synthesised materials was measured by means of Atomic Emission Spectroscopy with Induced
Coupled Plasma (ICP-AES) analysis collected in a Varian Vista AX system.
2.3. Catalytic tests in an up-flow fixed bed reactor
Fig. 1 depicts the experimental set-up used for the degradation of phenol aqueous
solutions by means of Catalytic Wet Hydrogen Peroxide Oxidation (CWHPO). The tubular
reactor was made of glass with inner diameter of 1.2 cm and 15 cm of length. The catalyst
particles were packed between glassy beads to enable a better distribution of the inlet solution
inside the catalyst bed and achieve the desired temperature. The operation conditions for the
evaluation of the activity and stability of the synthesized materials were set according to
preliminary studies with this experimental set-up [19]: i) reactor temperature of 80 ºC, ii) 1
mL/min of feed flow rate, iii) 1 g/L of phenol concentration and 5.1 g/L of H2O2 (stoichiometric
amount for the complete phenol mineralization, according to the reaction (1)) in the inlet
solution, and iv) 2.9 g of catalyst with a particle size between 1.6 and 2 mm.
Pump
FBR
Thermal bath
Cooler
Sample
Treated effluent TR
FC
TI TC
QA Feed
110 ºC
SET MENU
Pump
FBR
Thermal bath
Cooler
Sample
Treated effluent TR
FC
TI TC
QA Feed
110 ºC
SET MENU
Fig. 1.Experimental set-up of the up-flow fixed bed reactor for the catalytic wet hydrogen peroxide oxidation of phenol aqueous solutions.
Samples were hourly withdrawn from the treated effluent to determine Total Organic
Carbon (TOC), phenol and hydrogen peroxide concentrations, as well as iron leached out from
the catalyst to the treated effluent. TOC content of the solutions was analysed using a
combustion/non dispersive infrared gas analyser model TOC-V Shimadzu. Phenol conversion
was determined by means of HPLC chromatograph (Varian Prostar) equipped with a Waters
Spherisorb column and an UV detector adjusted at 215 nm, employing ultrapure water acidified
with H3PO4 up to pH 2.7 as mobile phase. Hydrogen peroxide conversion was determined by
iodometric titration. Iron content in the treated solution was measured by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES) analysis collected in a Varian VISTA AX
3. Results and discussion
3.1. Catalysts characterization
Table 1 shows the physicochemical properties of the synthesized powder and extruded
iron catalysts, as well as the silica materials used as supports for impregnation. The materials
have been classified in different groups depending on the silica support: mesostructured
SBA-15, mesoporous silica obtained via sol-gel and commercial extruded mesoporous silica acquired
from PQ Corporation. As can be seen, the first group corresponds to hexagonal mesostructured
SBA-15 materials, where iron was incorporated by direct synthesis (Fe2O3/SBA-15(DS)) and
incipient wetness impregnation (Fe2O3/SBA-15(I)). Additionally, physical mixture of hematite
iron oxide and the SBA-15 silica support (Fe2O3+SBA-15(PM)) was also prepared. The second
group includes mixed Fe2O3-SiO2 materials synthesized by simultaneous condensation of iron
and silica sources via the sol-gel method (Fe2O3/SiO2(DS)) or impregnation of mesoporous
silica sol-gel (Fe2O3/SiO2(I)). Finally, the third group is constituted by impregnated commercial
silica support (Fe2O3/SiO2-PQ(I)).
Table 1. Characterization data of iron-containing catalysts supported over silica materials.
Material
(Synthesis method) Support
Powder Extruded
SBET
(m2/g) (cmVp 3/g) Fe (wt.%) (mSBET 2/g) (cmVp 3/g) Fe (wt.%)
SBA-15
SBA-15
790 1.21 - - -
-Fe2O3/SBA-15(I) 549 0.78 15.8 333 0.53 12.4
Fe2O3/SBA-15(DS) 475 0.67 18.0 264 0.48 14.0
Fe2O3+SBA-15(PM) - - - 277 0.49 13.8
SiO2
SiO2
sol-gel
296 1.79 -
-Fe2O3/SiO2(I) 207 0.85 15.9 180 0.62 12.0
Fe2O3/SiO2(DS) 270 0.69 14.8 225 0.55 11.1
SiO2-PQ
SiO2 PQ
- - - 346 1.16
-Fe2O3/SiO2–PQ(I) - - - 252 0.87 15.4
Characterization data of powder materials evidence that catalysts based on SBA-15 silica
supports have higher specific surface areas than those supported over the silica synthesized via
nm) with hexagonal arrangement, which provides remarkable surface areas and pore volumes
(650-800 m2/g and 1-1.2 cm3/g, respectively). In contrast, silica sol-gel materials depict broader
pore size distributions and higher average pore diameter than SBA-15 based materials, which
lead to higher ratio between pore volume and specific surface area. As result of the iron
incorporation process over both silica supports, a significant reduction of surface area and pore
volume were evidenced for all the catalysts, although this fact was particularly more
accentuated for the incorporation of iron by direct synthesis over SBA-15 materials
(Fe2O3/SBA-15(DS)) [20]. Nevertheless, the powder catalysts synthesized over SBA-15
supports show higher specific surface areas regardless of the synthesis method (475 and 550
m2/g, for Fe
2O3/SBA-15(DS) and Fe2O3/SBA-15(I), respectively) than the counterparts
supported over non-ordered mesoporous silica (270 and 207 m2/g for Fe
2O3/SiO2(DS) and
Fe2O3/SiO2(I), respectively).
XRD patterns illustrated in Fig. 2 evidence typical diffractions of the hexagonal
arrangement of mesoporous materials for powder iron containing SBA-15 catalysts in the low
angle range, whereas iron catalyst prepared by gel method or impregnation over silica
sol-gel does not show any sign of mesoscopic order. At higher angles, clear diffraction peaks at 33,
36 and 42 º are observed, which are characteristic of crystalline hematite iron oxide particles,
except for Fe2O3/SBA-15(I) catalyst. This fact could be related to the small size of the
crystalline iron oxide particles which makes less visible to the X ray diffraction. The
comparison between XRD pattern of powder and extruded materials indicates that the
characteristic diffractions of hematite iron oxide particles are not affected by the agglomeration
process. Additionally, the hexagonal mesoscopic order of SBA-15 materials was also
maintained.
As can be seen in Table 1, the percentage of iron incorporated over the powder catalysts
ranged from 15 to 18 wt. %, whereas for agglomerated materials decreased up to 11-15 wt. %.
This reduction is consequence of the dilution effect related to the inorganic bentonite clay
incorporated into the extruded material during the agglomeration process. Moreover, the
the powder counterparts. This phenomenon is also attributed to the amount of bentonite clay
incorporated in the extruded material, which presents a low surface area (19 m2/g), as well as
the sintering process that take place during the final calcination step. Interestingly, the effect of
the sintering process seems to be more important in SBA-15-based iron catalysts, which suffer
40-45 % of surface area reduction, than iron catalyst supported over non-ordered silica sol-gel
materials (with reductions in the range 13-16 %). Nevertheless, iron containing catalysts based
on mesostructured SBA-15 silica matrices still exhibit higher specific surface areas. Pore size
distribution profiles shown in Fig. 3 for agglomerated catalysts confirm the characteristic pore
size distribution of mesostructured SBA-15-type materials with a narrow pore size distribution
centred at 8-9 nm, as compared to those shown by non-ordered mesoporous materials prepared
by the sol-gel method with a broader pore size distribution. Unlike the physical mixture of
Fe2O3+SBA-15(PM) which have shown the highest pore diameter (8.8 nm), the iron
incorporation over SBA-15 silica support by direct synthesis or impregnation have yielded a
slight reduction of the pore diameter up to ca. 7.7 and 8.2 nm, respectively. Regarding to iron
catalysts supported over non-ordered mesoporous silica, the sample Fe2O3/SiO2(DS) exhibits a
broader pore size distribution centred at ca. 15 nm as compared to the other two catalysts of this
group.
TEM images of extruded iron catalysts (Fig. 4) confirm the hexagonal arrangement of
SBA-15-based materials. In both groups of iron containing catalysts, mesostructured SBA-15
and non-ordered mesoporous silica materials, the iron particle size is strongly dependent on the
synthesis method and type of silica support. Interestingly, Fe2O3/SiO2(DS) catalyst, prepared by
the sol-gel method, evidence a remarkable dispersion of iron particles over the SiO2 support.
Unlike iron impregnated catalysts over non-ordered mesoporous silica sol-gel (Fe2O3/SiO2(I))
and silica-PQ (Fe2O3/SiO2-PQ(I)), impregnated Fe2O3/SBA-15(I) catalyst shows a better
dispersion of smaller iron particles over the silica framework. These data seem to be in
agreement with the small iron oxide particle sizes assumed from the X ray diffraction patterns.
The presence of agglomerating bentonite layers can be also observed together with the silica
(b)
Fe2O3+SBA-15(PM)
Fe2O3/SBA-15(I)
Fe2O3/SBA-15(DS)
25 30 35 40 45
2θ
1 2
Fe2O3/SiO2-PQ(I)
Fe2O3/SiO2(I)
Fe2O3/SiO2(DS)
2θ (º)
1 2
Fe2O3/SiO2(I)
Fe2O3/SiO2(DS)
SiO2
2θ (º)
25 30 35 40 45
2θ (a)
Fe2O3/SBA-15(I)
Fe2O3/SBA-15(DS)
SBA-15 In te ns ity (a .u ) (b)
Fe2O3+SBA-15(PM)
Fe2O3/SBA-15(I)
Fe2O3/SBA-15(DS)
25 30 35 40 45
2θ
1 2
Fe2O3/SiO2-PQ(I)
Fe2O3/SiO2(I)
Fe2O3/SiO2(DS)
2θ (º)
1 2
Fe2O3/SiO2(I)
Fe2O3/SiO2(DS)
SiO2
2θ (º)
25 30 35 40 45
2θ (a)
Fe2O3/SBA-15(I)
Fe2O3/SBA-15(DS)
SBA-15 In te ns ity (a .u )
Fig. 2. XRD patterns of (a) powder and (b) extruded iron-containing materials over mesostructured
SBA-15 and non ordered mesoporous silica supports.
1 10 100
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40
Fe2O3/SiO2(DS)
Fe2O3/SiO2(I)
Fe2O3/SiO2-PQ(I)
(b) d V /d lo g( D ) ( cc )/ g-n m )
Pore diameter (nm)
1 10 100
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40
Fe2O3/SBA-15(DS)
Fe2O3/SBA-15(I)
Fe2O3+SBA-15(PM)
(a) d V /d lo g( D ) (c c) /g -n m )
Pore diameter (nm)
1 10 100
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40
Fe2O3/SiO2(DS)
Fe2O3/SiO2(I)
Fe2O3/SiO2-PQ(I)
(b) d V /d lo g( D ) ( cc )/ g-n m )
Pore diameter (nm)
1 10 100
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40
Fe2O3/SBA-15(DS)
Fe2O3/SBA-15(I)
Fe2O3+SBA-15(PM)
(a) d V /d lo g( D ) (c c) /g -n m )
Pore diameter (nm)
Fig. 3. Pore size distribution of the extruded materials: (a) supported over SBA-15; (b) supported over non-ordered mesoporous SiO2.
Fe2O3
SiO2
1 µm
SBA-15
Bentonite Fe2O3
200 nm
Fe2O3+SBA-15(PM)
500 nm
Fe2O3
SBA-15 Fe2O3/SBA-15(DS)
200 nm
Fe2O3-SiO2
500 nm
SBA-15
Fe2O3
1 µm
SiO2
Fe2O3
Fe2O3/SiO2-PQ(I)
Fe2O3/SBA-15(I)
Fe2O3/SiO2(DS) Fe2O3/SiO2(I)
Fe2O3
SiO2
1 µm
Fe2O3
SiO2
1 µm
SBA-15
Bentonite Fe2O3
200 nm
Fe2O3+SBA-15(PM)
500 nm
Fe2O3
SBA-15
500 nm 500 nm
Fe2O3
SBA-15 Fe2O3/SBA-15(DS)
200 nm
Fe2O3-SiO2
500 nm
SBA-15
Fe2O3
1 µm
SiO2
1 µm
SiO2
Fe2O3
Fe2O3/SiO2-PQ(I)
Fe2O3/SBA-15(I)
Fe2O3/SiO2(DS) Fe2O3/SiO2(I)
3.2. Catalytic treatment of phenolic aqueous solutions in a fixed bed reactor
The catalytic performance of iron-containing materials for the wet peroxide oxidation of
phenol aqueous solutions was evaluated in terms of TOC reduction, phenol degradation and
hydrogen peroxide conversion. Likewise, the stability of the catalysts along the treatment was
also studied analyzing the iron concentration leached out from the catalyst within the treated
outlet effluent. Table 2 summarizes the results of the parameters monitored in the outlet stream
at steady-state after eight hours of time on stream for all the synthesized catalysts.
Table 2. Activity and stability of the different iron-containing catalyst under steady-state conditions.
Catalyst XTOC
(%) X(%)Phenol X(%)H2O2 η∗ [Fe](mg/L)leached pH
Fe2O3/SBA-15(I) 61 100 100 0.52 123 4.0
Fe2O3/SBA-15(DS) 64 100 100 0.56 14 4.3
Fe2O3+SBA-15(PM) 62 100 100 0.53 5 5.0
Fe2O3/SiO2(I) 64 100 100 0.54 20 4.2
Fe2O3/SiO2(DS) 52 100 100 0.44 11 4.0
Fe2O3/SiO2-PQ(I) 21 93 100 0.18 120 2.3
∗η = Theoretical H
2O2 consumption/ Real H2O2 consumption
It is noteworthy that phenol is completely degraded for all the catalysts regardless of the
silica support and synthesis method. Results of TOC evidence a high catalytic performance of
SBA-15-based iron catalysts with conversions above 60 %. In the case of sol-gel materials,
Fe2O3/SiO2(I) keep a similar TOC conversion (64 %), whereas Fe2O3/SiO2(DS) undergoes a
slight reduction up to 52 %. In contrast, impregnated commercial silica PQ (Fe2O3/SiO2-PQ(I))
shows a striking decrease of TOC degradation until ca. 25 %. The hydrogen peroxide is totally
consumed at the operation conditions under study for all the catalysts and with similar
efficiency (η≈0.55) except for the catalysts with lower TOC conversion previously mentioned
(Fe2O3/SiO2(DS) and Fe2O3/SiO2-PQ(I)). Values of oxidant efficiency around 0.5-0.6 indicate an
acceptable use of the oxidant. It must be pointed out that oxygenated by-products (mainly
carboxylic acids) formed at high TOC degradation rates are much more refractory than phenolic
Nevertheless, it is particularly remarkable the significant efficiency decrease for Fe2O3/SiO2
-PQ(I) (η = 0.18). The lowest oxidant efficiency of Fe2O3/SiO2-PQ(I) could be related to the
nature of the commercial silica or presence of metal traces that enable the decomposition of the
oxidant to other less active radical species, different of hydroxyl radicals.
Regarding iron leaching, it is important to note that catalysts prepared by impregnation
(Fe2O3/SiO2(I), Fe2O3/SiO2-PQ(I) and Fe2O3/SBA-15(I)) display higher loss of iron in the outlet
stream than those prepared by direct synthesis (Fe2O3/SiO2(DS) and Fe2O3/SBA-15(DS)). This
fact indicates weaker interactions between the iron species and the silica support for
iron-containing catalysts prepared by impregnation, which is not only contributed to faster catalyst
deactivation but also to additional metal pollution in the treated effluent, as it is the case of
Fe2O3/SBA-15(I) and Fe2O3/SiO2-PQ(I) catalysts with iron concentrations in the outlet stream
above 100 mg/L. The best stability of Fe2O3/SiO2(I), as compared to the other two impregnated
catalyst over mesostructured SBA-15 and mesoporous silica-PQ, indicates a stronger interaction
between iron particles and silica support. This can be related to the particular physicochemical
properties of the silica matrix for supporting small iron clusters. In this sense, the high density
of silanol groups that typically provides silica prepared via sol-gel, could be responsible of
stronger interlinking of the impregnated iron species over the silica framework.
Looking at the pH values, the decrease of the initial phenol aqueous solution pH (ca. 5.5)
is observed. This decrease is normally attributed to the formation of carboxylic acids as final
refractory by-products of the partial oxidation of phenol as well as protons by Fenton related
reactions [22]. Some studies have reported that metal iron leaching of Fenton-like catalysts can
be influenced by the solution pH in systems operating in batch conditions [12]. In this study for
the continuous treatment of phenol aqueous solution with different catalysts this type of
relationship was not evidenced.
The feasibility and durability of the iron catalysts for the continuous treatment of a phenol
aqueous solution can be also demonstrated by the results of TOC conversion and iron leaching
along the time on stream (Figure 5 and 6, respectively). TOC conversion profiles revealed times
mixture of Fe2O3+SBA-15(PM), which needs almost 7 hours. Although this enlargement of the
stabilizing time has not deeply studied, this phenomenon might be related to particular
diffusional constraints associated with the accessibility of the supported iron particles depending
on the silica support and the type of synthesis method. Nevertheless, it must be remarked the
stability of the TOC degradation rate, once the steady state is reached, up to 8 hours on stream
for all the catalyst, excepting Fe2O3+SBA-15(PM).
0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100 (a)
XTO
C
(
%
)
Time on stream (hours) 0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100 (b)
XTO
C
(
%
)
Time on stream (hours)
0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100 (a)
XTO
C
(
%
)
Time on stream (hours)
0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100 (a)
XTO
C
(
%
)
Time on stream (hours) 0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100 (b)
XTO
C
(
%
)
Time on stream (hours)
0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100 (b)
XTO
C
(
%
)
Time on stream (hours)
Fig. 5. TOC conversion profiles for the different iron-containing catalysts prepared by means of direct synthesis and incipient wetness impregnation: (a) -□- Fe2O3/SBA-15(I), -◄- Fe2O3/SBA-15(DS),
-○- Fe2O3+SBA-15(PM); (b) -▼- Fe2O3/SiO2(I), -◊- Fe2O3/SiO2(DS), -■- Fe2O3/SiO2-PQ(I).
According to the leached iron profiles (Figure 6), catalysts prepared by direct synthesis
Fe2O3/SBA-15(DS) and Fe2O3/SiO2(DS) as well as impregnated Fe2O3/SiO2(I) exhibited a good
behaviour with relatively low values, between 10 and 20 mg/L, and slight variation of the iron
concentration detected along the time on stream. Profiles of Fe2O3/SBA-15(I) and Fe2O3/SiO2
-PQ(I) catalysts, evidence a low durability for long operating times. Attending to the legal
discharge limit for iron concentration (10 mg/L), one of the main aims to apply successfully
heterogeneous catalytic systems is the reduction of iron leaching to lower values, which
promote higher durability of the catalyst. In this sense, the physico-chemical properties of the
materials prepared by direct synthesis, independently of the used support, allow improving the
catalytic stability, being consequently more interesting for this kind of applications.
Summarizing, catalysts prepared by direct synthesis over mesostructured SBA-15 and
of TOC reduction, and stability, measured as iron leached out to the treated effluent. These
materials provide activities above 50 % in terms of TOC conversion as well as reduced iron
leaching below 15 mg/L. Although mesostructured Fe2O3/SBA-15(DS) catalyst presents higher
values of TOC degradation (64 %) and hydrogen peroxide efficiency, the non-ordered
mesoporous Fe2O3/SiO2(DS) prepared via sol-gel is also considered as a potential candidate for
its application in industrial catalytic wet peroxide oxidation processes. The decreased catalytic
performance of Fe2O3/SiO2(DS) as compared to its counterpart over mesostructured SBA-15
support could be related to its either lower iron content (11.1 vs 14 %) or lower specific surface
area (225 vs 264 m2/g).
0 1 2 3 4 5 6 7 8
0 20 40 60 80 100 120 140 160 180 (a) [F e]le ac h ed ( m g/ L )
Time on stream (hours)
0 1 2 3 4 5 6 7 8
0 20 40 60 80 100 120 140 160 180 (b) [F e]le ac h ed ( m g /L )
Time on stream (hours)
0 1 2 3 4 5 6 7 8
0 20 40 60 80 100 120 140 160 180 (a) [F e]le ac h ed ( m g/ L )
Time on stream (hours)
0 1 2 3 4 5 6 7 8
0 20 40 60 80 100 120 140 160 180 (b) [F e]le ac h ed ( m g /L )
Time on stream (hours)
Fig. 6. Leached iron concentration for the different iron/silica catalysts prepared by means of direct synthesis and incipient wetness impregnation: (a) -□- Fe2O3/SBA-15(I), -◄- Fe2O3/SBA-15(DS),
-○- Fe2O3+SBA-15(PM); (b) -▼- Fe2O3/SiO2(I), -◊- Fe2O3/SiO2(DS), -■- Fe2O3/SiO2-PQ(I).
4. Conclusions
Different iron-containing catalysts based on hematite crystalline iron oxides supported
over mesostructured SBA-15 and non-ordered mesoporous silica were tested in an up flow fixed
bed reactor for the continuous catalytic wet peroxide oxidation of phenol aqueous solution.
SBA-15 silica support provides iron catalysts with narrower pore size distributions and higher
specific surface areas than non-ordered mesoporous silica. The agglomeration process do not
produce significant changes in the physicochemical properties of the precursor powder
catalysts, although in the case of mesostructured SBA-15 silica supports, a more accentuated
The activity of the iron containing catalyst in terms of TOC conversion and oxidant
efficiency was mainly affected by the type of silica support, whereas the iron resistance to be
leached out was principally dependent on the synthesis method. The iron incorporation in the
silica matrix by means of direct synthesis (co-condensation of iron and silica sources) has
provided higher stability of the supported iron species. In contrast, those catalysts prepared by
incipient wetness impregnation proved higher losses of iron in the treated aqueous solution (up
to 100 mg/L). Mesostructured Fe2O3/SBA-15 is shown as the best catalyst with a high catalytic
performance and reduced iron leaching for the treatment of phenolic aqueous solutions.
Non-ordered mesoporous Fe2O3/SiO2(DS) catalyst prepared via sol-gel in absence of organic
template was also considered an interesting candidate for its application in wet peroxide
oxidation processes. Both catalysts achieved a total phenol removal with TOC reductions above
50 % and relative low iron leaching (below 15 mg/L) operating for eight hours on stream.
Acknowledgements
The authors thank “Ministerio de Ciencia y Tecnología” for the financial support through
the project Consolider-Ingenio 2010 and “Comunidad de Madrid” through the project
P-AMB-000395-0505.
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